The invention relates to a method of set point control for steam temperatures for the start-up of a turbine and a steam generator in unit power plants.
When starting up steam turbines, in particular at initially low metal temperatures (cold start), provision must be made for suitable control of the steam throughput or steam temperature so that the stationary thermal stress in the thick-walled metal parts of the turbine do not exceed safe limits. On the other hand, the permissible values are to be utilized fully, so that the start-up time will be short and the energy losses kept low as are practical. Arrangements have become known which make it possible to fulfill these requirements (see for example Brown Boveri Mitteilungen 51 (1964), No. 3, p. 156-164).
The practice has shown that while the start-up process can be optimally regulated by such arrangements with respect to the turbine alone (see Brown Boveri Mitteilungen 51 (1964), No. 3, p. 186-194), this is not so for the entire system combination. If the steam temperatures are much above the metal temperatures, the load absorption of the turbo group is greatly decelerated by the probe control for a protracted time, in order then to reach the final value at increasing speed (see, e.g. Brown Boveri Mitteilungen 45 (1958), No. 7/8, p. 341, FIG. 6c). Not only is this load pattern undesirable from the viewpoint of the load distributor, but it also frequently leads to difficulties in the boiler control.
When starting up at steam temperatures which differ by a small amount from the metal temperatures but are low, the probe control alone would bring the turbo group to a high steam throughput quickly, but because of low enthalpy of the steam this does not lead to a corresponding load absorption and is not readily permissible because of too wet a steam in the end stages.
It is an object of the invention to avoid the disadvantages of the known solutions in particular in unit power plants. This is achieved by means which, for the purpose of optimal system start-up, coordinate the start-up of the turbo group and steam generator in such a way that during the start-up, at full utilization of the permissible stresses of the turbine, the load absorption of the turbo group and the inlet steam temperature are increased simultaneously and steadily, and more particularly so that both quantities reach their end values synchronously and in a time as short as is practically possible in view of any disturbing factors.
The invention will now be explained in greater detail with the aid of the one and only FIGURE given as an example.
As is evident from "Brown Boveri Mitteilungen" 45 (1958), No. 7/8, p. 339, upon the start-up of steam turbines, i.e., upon the warming up of thick-walled metal parts, uniform stresses will result during the entire time of this process based on the condition that the surface temperature TMO rises first at a discontinuity or "jump" and thereafter increases at an appropriate slope. The mean metal temperature TMM is thus represented by a slope type curve from the start.
According to the proposed method, a start-up probe is provided which measures the value TS = TMO - TMM, which value is representative of the stress of the turbine. The value TS is maintained at the desired value TSS by the acceleration regulator (load regulator) by control of the steam throughput Thus the heat flow QS flowing into the probe is also constant.
On this basis it is possible to calculate in a simple manner the desired value TES of the entrance steam temperature needed for the steam temperature set point control, with the aid of appropriate computing and measuring units.
If TA is the temperature drop between turbine inlet and probe measuring point, and TDM the temperature drop steam-to-metal, it follows that EQU TES = TMM + TSS + TDM + TA,
tmm being measured, and TSS and TA being quantities given by the machine data. TDM depends on the heat transfer steam-to-metal and on the heat flow QS flowing through the interface, which, as set forth above, is constant. According to "Brown Boveri Metteilungen" 45 (1958), No. 7/8, page 341. FIG. 5, p. 341, the heat transfer .alpha./.alpha.o is a function of the load. In a practical application a linear relationship can be assumed. If TDMN is the temperature drop steam-to-metal at the nominal load P.sub.n there follows for the load P: EQU tdm .apprxeq. tdmn .times. p.sub.n /P.
to avoid the sometimes very undesirable effect that initially a high steam temperature is demanded, then one passing through a minimum and only then again a rising steam temperature, P must be limited to the minimum value P.sub.MIN before insertion in the temperature set point calculation. Hence: EQU TES = TMM + TA + TDMN .times. P.sub.n /MAX (P, P.sub.MIN) (1)
according to the above, this is the desired value of the inlet steam temperature and suitable calculating units still to be described for realizing the above relation provide that, according to the proposed method with a uniform rise of the load absorption P, TES is also increased uniformly, so that both quantities reach their end values simultaneously.
In the proposed method there is to be calculated, in addition to the desired temperatures of the steam for the information of the load distributor, a prognosis at the start moment for the mean load variation in time.
This calculation occurs on the basis that the described set point control brings the steam temperature to its target value in the same period of time as the start-up regulation of the load. The probe value adjusted to the desired value TSS is proportional to the permissible mean metal temperature variation in time, GMZ; we have GMZ = TSS/ZKS where ZKS is a characteristic time constant determined by the dimensions and material constants of the probe. If the metal temperature TM must pass through the temperature difference TDEL during start-up, then the time interval necessary for this is t + TDEL/GMZ and hence the mean load variation in time in %/sec EQU BG = 100 .times. GMZ/TDEL
because of the temperature dependence of the determining material constants, the time constant ZKS and hence the time interval .DELTA. t for start-up are also dependent on the metal temperature. In the proposed method this fact is taken into consideration by calculating the mean of the mean metal temperature or more particularly its deviation ATM from a reference temperature, preferably 300.degree. C, introducing an appropriate correction of the permissible mean metal temperature variation in time referred to the reference temperature with the combined temperature coefficient TK of the material constants; this gives the prognosis BG for the mean load variation in time EQU BG = 100 .times. GMZ/[ TDEL .times. (1 + ATM .times. TK)] (2)
if a mean variation in time is calculated for the high-pressure and the medium-pressure cylinders, the smaller value is controlling.
As has been remarked above, this prognosis BG calculated at the starting moment for the mean variation of the load in time is developed first for the information of the load distributor. In the sense of the proposed method, the quantity BG can also be alternatively introduced advantageously into the acceleration (load) regulator. Taking into account the turbine stress, the load set point is influenced further by the quantities determined by the start-up probes.
If the load process is not disturbed (e.g. by unforeseen fluctuations of the steam temperatures), the load set point and hence the load increases with a precalculated slope (BG). In this case there is no superposition of the probes because during the entire load process the actual probe values are almost coincident with their desired values.